Femtosecond inscription of a spectral array of four fiber Bragg gratings at the same spot, using a single uniform phase-mask

An array of four fiber Bragg gratings (FBGs) is inscribed at the same spot with a single uniform phase-mask (PM). The inscription setup consists of an 800 nm femtosecond laser, a PM, a defocusing spherical lens and a cylindrical focusing lens. The wavelength tunability of the center Bragg wavelength is achieved by a defocusing lens, and by PM translation. An FBG is inscribed, followed by three cascaded FBGs, which are inscribed exactly at the same spot, only after a movement of the PM. The second order Bragg grating array at ~1.55 µm transmission spectra of these four FBGs is measured, showing a spectral shift of ~1.8 nm between each one, and a total spectral shift of ~5.4 nm. The transmission dip of each FBG is approximately −6 dB. The third order Bragg grating array at ~1.04 µm transmission spectra shows a transmission dip of approximately −3 dB, a wavelength separation of ~1.2 nm between each one, and a total wavelength shift of ~3.6 nm.